Rapid advances in signal processing and video production have made it possible to provide high-resolution video images. Generally, the display systems available that can reproduce the high-resolution video images are very expensive, do not provide a high quality image with sufficient brightness, or are limited in size.
One common type of video display is the CRT. To display a video image, sequences of frames are displayed very rapidly on the CRT screen. Each frame must be fully scanned by a single electron beam within a very short time period. For example, for a 60 Hz frame refresh rate, each frame must be scanned in less than 1/60 of a second. Because a frame is defined by a number of adjacent lines, each line must be scanned within a small fraction of the frame period, depending upon the size of the display. For example, in a standard VGA 640×480 format, each of the 480 lines must be scanned in less than 3.5 microseconds. Of course, higher-resolution formats (e.g., SVGA 800×600 or HDTV 1920×1080) have a greater number of pixels and lines, therefore requiring correspondingly faster line scans.
Additionally, as resolution and brightness requirements increase with better image quality, CRT-based projectors reach some physical limits and therefore, other ways to project video and computer information have been proposed and developed.
Laser TV projectors have been proposed in which red, green, and blues lasers are individually modulated and combined to generate a full color image on a projection screen. In light-modulation projectors, laser radiation is modulated in a modulator array that switches individual display elements (pixels) on or off. Liquid crystal display (LCD) panels are common light modulators. Other modulators, such as acousto-optic modulators (AOMs), deformable micro-mirrors (DMDs), and microelectromechanical system (MEMS) techniques are also available.
Projection TVs are generally not able to reproduce images with the same clarity, brightness and contrast as large LCDs and plasma screen displays, but they are less expensive. All such displays have scanning systems, comprising a horizontal scanning unit and a vertical scanning unit, to cause an image to be displayed on a screen. Generally, the horizontal scanning unit must execute the horizontal image signal scans at a much faster rate than the vertical scanning unit.
A conventional raster scanning system uses a rotating polygon mirror scanner as the horizontal scanning unit and a galvanometer mirror as the vertical scanning unit, both of which have limited bandwidth (i.e. scanning rate). The rotating polygon mirror has been used reliably for many years, but has inherent limitations in the module size of the polygon mirror and the scanning rate. For example, for an HDTV application the rotating speed of the polygon mirror would be approximately 100,000 rpm. This requires a bulky and expensive polygon scanner module.
Another approach uses multiple beams with multiple scanners to scan multiple image areas to form a whole image. Specifically, an image is divided into quadrants and each scanner is responsible for a quadrant portion of the image. However, such an approach makes for very complicated control electronics to maintain each of the different scan areas in relation to the others and the device tolerance margins must be extremely tight. Additionally, the image quality deteriorates at the border of neighboring image areas. Other approaches achieve multiple image area scans using multiple beams and a single scanner. However, the same neighboring border image artifact quality problems arise. Additionally, each of the multiple beams requires a separate modulator, which leads to complicated control systems. Furthermore, vertical scanning is difficult and a wide-angle scanner is still required to increase the size of the display.
Generally, the resolution of raster scanning projection displays directly translates into a a product of θD, where θ is the maximum scanning angle of the scanner in degrees and D is the beam width or mirror size in millimeters. Therefore, the higher the image resolution, the greater the θD value required.
For example, the horizontal scanner for drawing an HDTV image requires a θD of approximately 22.5 degree-mm. Thus, if the scanning angle θ were 8 degrees, the D would need to be approximately 2.8 mm, and if the scanning angle θ were 4 degrees, the D would need to be approximately 5.6 mm.
MEMS scanners can improve the scanning rate but have limited scanning angles, thus limiting the size of the display screen to achieve high clarity and contrast. Also, MEMS devices used as a laser scanner provide the potential of competitive cost and high frequency scanning capability. However, the scanning angle and the scanner size are in a trade-off relationship in the MEMS device under fixed characteristic frequency conditions. To preserve the characteristic frequency of the MEMS scanner for a given image resolution, the scanning angle should be decreased if the scanner size is increased according to the physical principle of rigid body's harmonic oscillator model.
The trade-off relationship may be explained by study of the following equations:
            ax      +      bx      +      cx        =    N        N    =                  N        o            ⁢              ⅇ                  ⅈ          ⁢                                          ⁢          ω          ⁢                                          ⁢          t                          x    =                  x        o            ⁢              ⅇ                  ⅈ          ⁢                                          ⁢          ω          ⁢                                          ⁢          t                                x      o        =                            N          o                                                    (                              c                -                                                      ω                    2                                    ⁢                  a                                            )                        +                          i              ⁢                                                          ⁢              ω              ⁢                                                          ⁢              b                                          =                                    N            o                                                                                                    a                    2                                    ⁡                                      (                                                                  ω                        o                        2                                            -                                              ω                        2                                                              )                                                  2                            +                                                (                                      ω                    ⁢                                                                                  ⁢                    b                                    )                                2                                                    ⁢                  ⅇ                      ⅈ            ⁢                                                  ⁢            Δ                                          ω      o      2        =          c      /      a      Where x is the scanning angle, ωo is the characteristic frequency and c is the spring constant of the hinge of the MEMS scanning mirror, a is the inertia moment, b is a damping factor, c is a spring constant and N is the driving force. Scanner size is closely related to the inertia moment a. Assuming that ωo is maintained regardless of increases in the mirror size, then the inertia moment a will be increased and the spring constant c should be increased by the same fractional ratio as the inertia moment a. This results in the decrease of the scanning angle when the driving force N and the damping factor b are fixed.
What is needed is to develop a method and apparatus, which can increase the scanning angle without the trade-off with the mirror size. This would be especially useful for large, high-resolution, rear or front projection TVs to enable a clearer picture and better manufacturability.